<!DOCTYPE html>
<html lang='en'>
<head>
<meta charset='UTF-8'>
<meta name='viewport' content='width=device-width, initial-scale=1.0'>
<title>Document</title>
</head>
<body>

  <div id='app'>
    <input type="text" v-model="firstName">
    <input type="text" v-model="lastName">
    =>
    <input type="text" v-model="fullName">
  </div>

<script src='https://cdn.bootcdn.net/ajax/libs/vue/2.6.12/vue.js'></script>
<script>
  const app = new Vue({
    el: '#app'
    ,data: {
      firstName: ''
      ,lastName: ''
      ,fullName: ''
    }
    ,methods: {

    }
    ,watch:{
      firstName(newVal) {
        this.fullName = newVal + '·' + this.lastName;
      }
      ,lastName(newVal) {
        this.fullName = this.firstName + '·' + newVal;
      }
    }
  })
</script>
</body>
</html>